An electron device which requires size reduction and high function, such as a mobile telephone or the like generally uses constitution in which individual functional modules such as a CCD camera, a display panel, and the like are connected to a main electronic circuit module provided for a rigid substrate through a film-shaped flexible substrate. As methods of connecting a terminal provided on this flexible substrate to a circuit electrode of the rigid substrate, there have been known a method of soldering this terminal to the circuit electrode by a solder precoating method (refer to, for example, JP-A-6-85454), and a method of electrically connecting the terminal and the circuit electrode by means of ACF (anisotropic conductive film) (refer to, for example, JP-A-11-233912 and JP-A-11-167971).
In the solder precoating method, a solder part is previously formed on the circuit electrode by tinning, and the flexible substrate is press-heated on the rigid substrate, whereby the solder part is melted and the melted solder part solders the terminal to the circuit electrode. Hereby, the terminal and the circuit electrode electrically conduct, and the flexible substrate is mechanically secured onto the rigid substrate at the solder joint portion.
Further, in the method using the ACF, thermosetting resin containing conductive particles is supplied onto the circuit electrode, and the flexible substrate is heat-bonded to the solid substrate, whereby the circuit electrode and the terminal are brought into contact with each other with the conductive particles between them, and the thermosetting resin thermosets. Hereby, the circuit electrode and the terminal electrically conduct, and the flexible substrate and the rigid substrate are bonded to each other by the thermosetting resin that has hardened.
Further, in the method using the ACF, a method which uses solder particles as the conductive particles has been proposed (refer to, for example, JP-A-11-176879 and Japanese Patent No. 3417110). In this method, the solder particles put between the circuit electrode and the terminal melt by thermocompression bonding thereby to solder the circuit electrode and the terminal to each other, whereby the circuit electrode and the terminal electrically conduct.
However, the related arts indicated in the above related art references have the following disadvantages caused by features of their respective methods. Firstly, in the solder precoating method, addition of the step of previously forming the solder part on the circuit electrode causes the increase of cost in the production process. Further, there are disadvantages in joint reliability such that: narrow-pitching of the circuit electrodes causes readily a bridge in which the melted solder hardens in a state where it is joined together between the adjacent electrodes in the soldering time; and limitation of the precoating solder quantity due to the narrow-pitching makes it difficult to sufficiently secure the joint keeping power required after the connection.
Further, the method using the ACF has disadvantages such that: material cost of the ACF itself is expensive; and since thermocompression bonding for connection requires long bonding time, productivity is low and cost reduction is difficult. Further, since the terminal and the circuit electrode conduct through the conductive particles put in the contact state, electric resistance is high at the connection part, so that it is difficult to secure connection reliability.
Further, since a high bonding load (10˜20 MPa) is required in order to bring well the conductive particles into contact with the terminal and the circuit electrode, a large limitation exists for connection objects to which the connection by the ACF can be applied. Namely, in case that the rigid substrate is a multilayered substrate having inner wiring, or a double sided mounting substrate in which projections of electronic components mounted on the rear surface of the connection surface exist, stable backup capable of sufficiently supporting the bonding load is difficult, so that it is difficult to apply the connection by the ACF to such the substrate.
In order to solve the above problems in such the solder precoating method and connection method by the ACF, the method of using the solder particles as the conductive particles has been proposed as indicated in JP-A-11-176879 and Japanese Patent No. 3417110. However, in these related arts, there are the following problems caused by the shape of a solder part formed after the solder particles have soldered the terminal to the circuit electrode.
Namely, in case the solder particles are used as the conductive particles, the solder particles melt in a thermocompression bonding step. Therefore, regarding the shape of the solder part formed by cooling and hardening of the solder after the thermocompression bonding, a contact angle in the joint surface to the terminal or the circuit electrode becomes an obtuse angle, so that sharp shape-discontinuities are easy to be formed. Such the shape-discontinuities causes fatigue strength for heat cycle in a use state of the electronic components to lower greatly, with the result that connection-reliability is impaired. Thus, the conventional electronic component soldering structure which is applied to the electronic components such as the film-shaped substrate and the like has a problem that it is difficult to realize electronic component soldering structure capable of securing high connection-reliability at low cost and with high productivity in relation to various kinds of electronic components.